This application claims priority benefits under 35 USC xc2xa7119 on the basis of Japanese Patent Application No. 2000-99543, the disclosure thereof being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a throwaway tip for a drill for drilling, and to a holder for holding the same.
2. Description of Related Arts
Drills for drilling include solid type drills which have a unitary structure, and so-called throwaway-tipped drills which have a throwaway tip having a cutting edge at a front edge thereof and adapted to be attached to a holder by a screw or the like in a removable and replaceable manner.
The throwaway-tipped drills include double tip type drills for drilling a hole of a relatively great diameter (see, for example, Japanese Unexamined Patent Publication No. 10-29108 (1998)), and single tip type drills for drilling a hole of a relatively small diameter (see, for example, Japanese Unexamined Patent Publication No. 11-188518 (1999)).
In the case of the latter single tip type drills, a tip is inserted into a pocket defined between a pair of holder pieces of a holder and held between the pair of holder pieces. The tip is fixed to the holder by inserting a fixing screw into a through-hole of one of the holder pieces and a through-hole of the tip and then screwing a front end portion of the fixing screw into a threaded hole of the other holder piece.
With a recent demand for reduction of a drill diameter, the following problems associated with the diameter reduction are expected to arise.
A force is exerted on the respective holder pieces by the tip to twist the holder pieces in the direction of rotation during a drilling operation. This tends to expand a gap between the pair of holder pieces. Where the holder pieces each have a reduced thickness due to the diameter reduction, the gap between the holder pieces is more easily expanded.
With the reduction of the drill diameter, the diameter of the fixing screw should significantly be reduced. Where the drill has a nominal diameter of 10 mm, for example, the fixing screw needs to have a diameter of about 2.1 to 2.2 mm. This is because, if the fixing screw has a greater diameter, the through-holes of the tip and the holder piece for the insertion of the fixing screw each have a correspondingly greater diameter, thereby reducing the strength of the tip and the holder piece. The reduction in the strength should be prevented.
However, the tip is heavily pressed against the bottom of a drilled hole along the rotation axis during the drilling operation and, therefore, subjected to a strong thrust reaction force. This thrust reaction force is received by the smaller-diameter fixing screw. Hence, there is a possibility that the smaller-diameter fixing screw cannot withstand a shearing stress occurring due to a heavy thrust load. Further, the expansion of the gap between the holder pieces cannot be prevented.
It is therefore an object of the present invention to provide a throwaway tip which can stably be held even if it is subjected to a resistance during a drilling operation.
In accordance with a preferred embodiment of the present invention to achieve the aforesaid object, there is provided a throwaway tip which is adapted to be received in a pocket of a holder and clamped by a pair of holder pieces defining the pocket therebetween, the throwaway tip comprising: a substantially planar base; a cutting edge provided on a front edge of the base; a pair of holder bearing faces respectively provided on sides of the base to be respectively brought into abutment against the pair of holder pieces; and a rear bearing face provided on a rear side of the base to be received by a bottom of the pocket; wherein the rear bearing face is substantially orthogonal to the holder bearing faces.
In this embodiment, the rear bearing face is substantially orthogonal to the holder bearing faces. Therefore, even if a thrust force is exerted on the tip to press the rear bearing face of the tip against the bottom of the pocket, there is no possibility that the thrust force is converted into a force which forces the pair of holder pieces of the holder away from each other.
Since this embodiment employs such a layout that the rear bearing face of the tip is brought into abutment against the bottom of the pocket, the tip can flexibly be laid out with the rear bearing face thereof being orthogonal to the rotation axis or inclined at a predetermined angle with respect to the rotation axis. In addition, the bearing face is allowed to have a greater area so as to assuredly bear the thrust load. Thus, the tip subjected to the thrust resistance during the drilling operation can be held as stably as possible.